PCBs AND ASSOCIATED AROMATICS 947
Electrocoagulation has been applied to the removal of
contaminated soil washings, waste oils and the enhanced
dewatering of sludges. The applicability of the technique to
soils which have been solvent extracted to remove PCB con-
tamination is not yet known.
Solvent Extraction Three extraction techniques are used
to clean soil: water washing, solvent extraction and air strip-
ping. The first two methods are applicable to the remediation
of soils contaminated with PCBs.
Water washing alone is not effective in the removal of
hydrophobic contaminants such as PCBs. However, when
surfactants are added to the water washes laboratory studies
have shown an extraction efficiency of more than 90%.
Water washes used without added surfactant were found
to reduce the PCB concentration in a sample of soil by only
about 13%. The action of the surfactant is to reduce the
adsorptive bonding strength of the PCBs on the soil particles
with the result than 90% PCB removal was achieved.
Approximately 95% of the surface area of a soil is rep-
resented by fine particulates. The fines represent about 15%
of silt and 8% of clay. Therefore, a sieving process alone
is able to provide a particulate mass which contains a large
fraction of the total PCB contamination. The EPA Mobile
Soil Washing System (MSWS) was designed^163 around the
particle size concentration effect.
Solvent extraction with organic solvents tends to be more
effective than water washing with surfactant solutions. Two
general techniques are applied: (1) immersion extraction and
(2) leaching.
An immersion extractor is simply a tank containing
stirred solvent into which the soil is suspended and mixed.
When the extraction equilibrium has been established, the
solids are separated either by settling or centrifugation and
the solvent is passed down the treatment train to be decon-
taminated and recycled.
Leaching extraction is a process in which solvent is
allowed to percolate through the soil contained in a screened
bottom tank. The solvent desorbs and dissolves the PCB
contamination and is collected at the bottom of the tank.
A series of beds of soil are treated in a countercurrent fashion
to increase the extraction efficiency.
Critical fluid extraction is a method in which liquified
hydrocarbon gases such as propane and butane are used as
solvents for separating organic contaminants from soils,
sludges and sediments. Feed material is typically screened
to remove particles which are greater than 1/8 diameter and
then mixed with water to provide a pumpable slurry. The
technology has been applied on a mobile 60-tpd treatment
scale. A 98% PCB reduction (164) has been achieved from soil
initially contaminated with 350 ppm PCB.
A basic extractive sludge treatment uses aliphatic amines,
typically triethylamine, (TEA) as the solvent to remove
contaminants, including PCBs. TEA has the interesting
property that below 15°C it can simultaneously solvate oils
and water. Above 15°C, water becomes immiscible and
separates from the oil and solvent. The process has been
designed to remove water and organics at low tempera-
tures, separate the water from the organic phase at highertemperatures, and recover the solvent in high yield through
distillation. A 70-tpd unit has been demonstrated^165 on PCB
contaminated soils.
Physico-chemical Adsorption Activated carbon is a
strong adsorbent for PCBs because of its structure. Powdered
carbon which has a very high surface area to mass ratio is
used. The long-term physical and chemical stability of acti-
vated carbon added to soil systems is unknown but the mate-
rial tends to erode with the soil and also become distributed
in the environment as particulate emissions.
Carbon beds have been used to adsorb PCBs from water
as well as oils. The system acts as a PCB concentrator which
is usually then sent for incineration.
The following Table 56 shows a number of innovative
technologies which are being developed for the remediation
of contaminated soils.
The wide variety of technologies in the above table
have been applied to the PCB decontamination of soils with
varying degrees of success. The methods can be separated
into two groups. One group requires the use of large energy
sources and need a significant amount of setup time but are
relatively less selective with regard to the contaminants to be
treated. The second group tend to be more mobile, use much
less energy and apply a second process to the soil extract to
treat a specific group of contaminants.Electrochemical Treatment Processes for
Excavated SoilsThis section will discuss electrochemical methods which
have been used for the dechlorination of PCBs and focus on
data obtained with a method which appears to have wide-
spread applicability. Other electrochemical methods will be
discussed briefly.
A Westinghouse patent deals with an electrochemi-
cal method for the treatment of PCBs in which hydrogen
peroxide is generated by an alternating current field passed
through a series of packed beds of conductors. The high and
low conductivity beds are placed adjacent to one another
and each has separately controlled pairs of electrodes. The
low conductivity bed contains conductive particles such as
carbon or non-conductive particles coated with oxides such
as MnO 2 or PbO 2. The proportion of particles in the more
conductive bed is about 30% oxide coated particles with the
balance as conducting and/or absorptive particles. A series
of beds was constructed in the effluent flow path and 29
volts/12 amps passed into each of the less conducting beds
while 10 volts/15 amps was passed into each of the more
conductive beds. The electrodes were cooled so that the pro-
cess temperature could be minimized. The PCB concentra-
tion was reduced from 120 ppb to 1.5 ppb.
Controlled potential electrolysis has been investigated
as a method for the dechlorination of PCBs. Radical anions
of such compounds as anthracene, 9,10-diphenylanthra-
cene and phthalonitrile in dimethylformamide solution are
formed at a mercury electrode surface. The radical anions
react with PCBs in solution to produce chloride in much
the same way as the mechanism of action of organometallicC016_003_r03.indd 947C016_003_r03.indd 947 11/18/2005 1:12:46 PM11/18/2005 1:12:46 PM